1. Field of the Invention
The present invention relates generally to a combustor of a gas turbine, and more particularly to a combustor structured such that uniformity of combustion air intake is attained so as to enhance combustion efficiency and combustor cooling ability, as well as a fitting structure of structural portions which are less durable against thermal stress, such as a combustor main swirler or a pilot cone. They are improved so as to not be influenced by high temperature, whereby overall efficiency of the gas turbine combustor is enhanced in view of recent tendencies of higher temperature combustion gas. The present invention also relates to a combustor of a gas turbine having reduced combustion vibration.
2. Description of the Prior Art
FIG. 20 shows a structural arrangement of a representative gas turbine combustor and surrounding portions thereof in the prior art. In FIG. 20, numeral 20 designates a combustor, which is provided in a turbine casing 50. Numeral 21 designates main fuel nozzles provided in plural pieces in a circumferential direction the combustor and is to be supplied with a main fuel of oil or gas. Numeral 22 designates a pilot fuel nozzle, which is provided in a central portion of the plural main fuel nozzles 21 for igniting the main fuel nozzles 21. Numeral 23 designates a combustion chamber, and numeral 24 designates a tail tube, from which a high temperature gas produced in the combustion chamber 23 is led into a gas turbine. Numeral 62 designates a compressor, numeral 63 designates an air outlet, numeral 64 designates an air separator for supplying gas turbine blades with outside air for cooling thereof, numeral 65 designates a gas turbine stationary blade and numeral 66 designates a gas turbine moving blade.
In the combustor constructed as mentioned above, air 40 coming from the compressor 62 flows into the turbine casing 50 via the air inlet 63 and further flows into the combustor 20, for effecting combustion, from around the combustor 20 through spaces formed between stays, described later, as air shown by numerals 40a, 40b. In the flow of the air 40 at this time, there arises differences in the flow rate and pressure between the air 40a which is near the air outlet 63 or the compressor 62 and the air 40b which is far from the air outlet 63 or the compressor 62. This causes a non-uniformity in the air flow entering the combustor 20 according to the circumferential directional position thereof, with the result that a biased flow of air arises in an inner tube, described later, in the combustor 20, causing a non-uniformity of fuel flow as well, which leads to an increase of NO.sub.x formation.
FIG. 21 is an enlarged schematic view of the gas turbine combustor of FIG. 20. In FIG. 21, there are shown several structural portions having shortcomings to be addressed. That is, an (X-1) portion and an (X-2) portion are air intake portions into the fuel nozzles, an (X-3) portion is a main swirler fitting structural portion, an (X-4) portion is a pilot cone fitting structural portion and an (X-5) portion is a tail tube cooling structural portion. There are problems to be solved in the respective portions. Such problems as exist in the present situation will be sequentially described below.
The air intake portion (X-1) will be described first. FIG. 22 is a cross sectional view of a top hat type fuel nozzle portion of a prior art gas turbine. In FIG. 22, the air 40a, 40b coming from the compressor flows into the combustor 20 for effecting a combustion from around the combustor 20 through spaces formed between supports 25 provided in the combustor 20. Between the air 40a which is near the compressor and the air 40b which is far from the compressor, there are differences in the flow passages themselves and the shapes thereof, which causes a non-uniformity in the flow rate of the air flowing into the combustion chamber 23 according to the circumferential directional position thereof so as to cause a biased flow of the air. By this biased flow of the air, fuel flow also becomes non-uniform in the combustion chamber, and NO.sub.x formation increases. It is needed, therefore, that the air flow into the combustor be uniform in the circumferential direction.
Also, in the combustor of FIG. 22 which is of the top hat type, there is fitted to the turbine cylinder 50 an outer tube casing cover 51 for covering a portion where the fuel nozzles are inserted. On the other hand, in the combustor of FIG. 20, the air intake portion is arranged in a space formed by a cylindrical casing of the turbine casing 50. In the example of FIG. 22, a portion surrounding the supports 25 as the air intake portion is covered by the cylindrical outer tube casing cover 51. The outer tube casing cover 51 is of a hat-like shape which projects toward the outside. In this type of combustor, a central axis 61 of the outer tube casing cover 51 of the turbine casing 50 and a central axis 60 of the combustor do not coincide with each other, and the combustor is fitted to the outer tube casing cover 51 so as to incline slightly thereto. Although a detailed explanation of the reason therefor is omitted, while the combustion gas flowing through the inner tube and the tail tube is led into a gas turbine combustion gas path, the temperature distribution of the gas flow is needed to be made as uniform as possible. In order to realize an optimized temperature distribution according to the manner in which the combustor is fitted, the central axis 60 of the combustor is inclined slightly relative to axis 61 of the outer tube casing cover 51.
In the portion surrounding the supports 25, as the air intake portion in such combustor, there are differences along the circumferential direction in the space areas formed by the outer tube casing cover 51 and the supports 25, and while the quantity of intake air is varied in this way, there is still a non-uniformity of the intake air. In this type of combustor, while the outer tube casing cover 51 functions as a correcting tube to some extent, so that there is obtained some correction effect of the air flow coming into the combustor, as compared with the combustor of FIG. 20, the air takes turns at the air intake portion surrounding the supports 25 to flow into the nozzle portion. This causes a non-uniformity of the air flow, and hence improvement so as to realize a more uniform flow of the air is desired.
Next, a problem existing in the air intake portion (X-2) will be described. FIG. 23 is a side view of an inner tube portion of the combustor 20 of FIG. 20. In FIG. 23, a high temperature combustion gas 161 flows through the inside of an inner tube 28. In a circumferential surface of the inner tube 28, which is exposed to the high temperature gas, there are provided a multiplicity of small cooling holes (not shown). Air flowing through these cooling holes cools the inner tube 28 to then flow out to be mixed into the combustion gas flowing inside the inner tube 28. On the other hand, there remains an unburnt component of fuel in the combustion gas flowing through the inner tube 28, increasing the NO.sub.x formation, and hence it is necessary to sufficiently burn the unburnt component. For this purpose, there are provided in the circumferential surface of the inner tube 28, air holes 10-1, 10-2, and 10-3 formed in three rows, with six air holes in each of the rows. The six air holes of each row are arranged with equal intervals between them in the circumferential direction of the inner tube 28, as shown in FIG. 23.
In the inner tube 28 constructed as above, the combustion gas 161 produced by the main fuel nozzle 21 flows through the inner tube 28 to flow to the tail tube 24. For combustion of the unburnt component of fuel contained in the high temperature combustion gas 161, air 130 is led into the inner tube 28 through the first row of air holes 10-1 and the second row of air holes 10-2. Further, air 131 is led into the inner tube 28 through the downstream third row of air holes 10-3 for combustion of the unburnt component still remaining unburnt.
The air entering the combustor 20 comprises three portions, that is, the air used for combustion at the nozzle portion of the combustor, the air entering the inner tube 28 for cooling thereof through the small cooling holes and the air 130, 131 flowing into the inner tube 28 through the air holes 10-1, 10-2, and 10-3. Where the total quantity of these three portions of the air is 100%, as one example in a prior art combustor, the quantity of the air flowing through the air holes 10-1, and 10-2 is about 14% each, and that of the air flowing through the air holes 10-3 is about 19 to 20%. If the respective quantities are expressed in a ratio for the air holes 10-1, 10-2 and 10-3, it is expressed as approximately 1:1: (1.3 to 1.4). That is, the air quantity entering the inner tube 28 through the downstream air holes 10-3 is largest. But if the air quantity entering through the air holes 10-3 becomes excessive, it remains unused for combustion, and cools flames of the high temperature combustion gas to thereby cause a colored smoke.
Next, a problem existing in the main swirler portion (X-3) will be described. In a prior art multiple type premixture combustor of a gas turbine, a pilot swirler is provided in a center thereof and eight pieces of main swirlers are arranged therearound. Each of the main swirlers is fixed by welding to an inner wall of the combustor via a thin fixing member of about 1.6 mm thickness. FIG. 24 is a cross sectional side view showing a swirler portion and a pilot cone portion of the type of combustor in the prior art and FIG. 25 is a partial view seen from plane H--H of FIG. 24. In FIGS. 24 and 25, numeral 20 designates a combustor, numeral 31 designates a pilot swirler provided in a center of the combustor 20 and numeral 33 designates a pilot cone fitted to an end of the pilot swirler 31. Numeral 32 designates a main swirler, which is arranged in eight pieces around the pilot swirler 31. Numeral 34 designates a base plate which is formed in a circular shape and has its circumferential portion fixed by welding to the inner wall of the combustor 20. In the base plate 34, there is provided a hole in a center portion thereof through which the pilot swirler 31 passes to be supported. Also provided are eight holes around the hole of the center through which the main swirlers 32 pass so as to be supported.
Numeral 35 designates metal fixing members, which are each formed of a metal plate and is interposed to fix each of the eight main swirlers 32 to the inner circumferential wall of an end portion 36 of the combustor 20 by welding. As shown in FIG. 25, the main swirlers 32 are fixed to the inner circumferential wall of the end portion 36 of the combustor 20 via the fixing metal member 35. Although omitted in the illustration, a main fuel nozzle has its front end portion inserted into the main swirler 32 and a pilot fuel nozzle has its front end portion inserted into the pilot swirler 31. Main fuel injected from the main fuel nozzle mixes with air coming from the main swirler 32 to be ignited for combustion by a flame, the flame being made by pilot fuel coming from the pilot fuel nozzle together with air coming from the pilot cone 33 of the pilot swirler 31. The mentioned combustor 20 is arranged in several tens of pieces, 16 for example, in a circle around a rotor in a gas turbine cylinder for supplying therefrom a high temperature combustion gas into a gas turbine combustion gas path for rotation of the rotor.
In the gas turbine combustor so made as a welded structure, a deformation occurs due to vibration or thermal stress in operation so as to cause cracks in the welded portion of the metal fixing member 35. This requires frequent repair work to replace the fixing metal member 35 or carry out additional welding work. In the fitting portion of the metal fixing member 35, there is only a narrow space for welding work, creating a bad condition for performing a satisfactory welding. As such, a high level of skill of the workers is required. Also, in making the welded structure, a fine adjustment in fitting is difficult, which restricts maintaining accuracy. That is, there is a problem in the work accuracy in making the welded structure.
Next, a problem existing in the pilot cone portion (X-4) will be described. In the combustor 20 described with respect to FIGS. 24 and 25, the main fuel nozzle is inserted into the central portion of the main swirler 32, and main fuel injected from the main fuel nozzle and air coming from the main swirler 32 are mixed together to form a premixture. On the other hand, the pilot fuel nozzle is inserted into the central portion of the pilot swirler 31, and pilot fuel injected from the pilot fuel nozzle together with air coming from the pilot swirler 31 burns to ignite the premixture of the main fuel for combustion in a combustion tube, which includes an inner tube and a connecting tube, to thereby produce the high temperature combustion gas.
FIG. 26 is a partial detailed cross sectional view of a fitting portion of the pilot cone 33 of FIG. 24. In FIG. 26, a cone ring 38 at its one end is fitted to an outer wall of the pilot cone 33 by welding W2. The cone ring 38 at the other end is fitted to a fitting member 39b, which is an integral part of a base plate 39, by welding W1. The pilot cone 33 is inserted into a cylindrical portion 39a of the base plate 39 and fixed to the base plate 39 by welding W3. An end portion 31a of the pilot swirler 31 is inserted into the pilot cone 33 to be fitted to the pilot cone 33 by welding W4. In the welding W4, a black arrow in FIG. 26 shows a direction in which the welding is carried out. Thus, the pilot cone 33 is fitted to the base plate 39 via the cone ring 38 by welding W3 and the pilot swirler 31 is fitted to the pilot cone 33 by welding W4. Hence, the base plate 39 fixes the central pilot swirler 31, the pilot cone 33 and the eight pieces of the main swirlers 32 by welding, as mentioned above, to support them in a base plate block.
Fitting work procedures of the mentioned welded fitting structure have the cone ring 38 first fitted around the fitting member 39b of the base plate 39 by welding 1, and then the pilot cone 33 is fitted to the cone ring 38 by welding W2. The pilot cone 33 is then fitted to the base plate 39 by welding W3 which is done around an end portion of the pilot cone 33. Thereafter, the pilot swirler 31 is inserted into the end portion of the pilot cone 33 to be fitted to the pilot cone 33 by welding W4 to be done therearound. Thus, in case the pilot cone 33 is to be uncoupled in the welded structure, the weldings W2, W3 and W4 need to be detached. But in the spaces around the weldings W2 and W3, there are arranged the main swirlers 32, making the work space very narrow. This results in the need to disassemble the entirety of the base plate block. In this situation, the accuracy of the welding is deteriorated and becomes easily influenced by the thermal stress of the high temperature gas.
As the pilot swirler 31 and the pilot cone 33 are continuously influenced by the high temperature combustion gas, and the base plate block is made with a thin plate structure, as mentioned above, cracks easily arise due to strain caused by the thermal stress. This necessitates frequent repair work with a high level of welding skill, and thus an improvement of such welded structure is desired.
Next, a problem existing in the tail tube cooling portion (X-5) will be described. In the recent tendency toward higher temperature gas turbines, a combustor is being developed in which the combustion gas reaches a high temperature of about 1500.degree. C., and the cooling system thereof is being tried to be changed to a steam type cooling system from an air type cooling system. FIG. 27 is an explanatory view showing a tail tube cooling structure in a representative gas turbine combustor in the prior art, which has been developed by the present applicants, wherein FIG. 27(a) is an entire view, FIG. 27(b) is a perspective view showing a portion of a tail tube wall and FIG. 27(c) is a cross sectional view taken on line J--J of FIG. 27(b). In FIG. 27(a), numeral 20 designates a combustor, which comprises a combustion tube and a tail tube 24. Numeral 22 designates a pilot fuel nozzle, which is arranged in a central portion of the combustion tube, and numeral 21 designates main fuel nozzles provided in eight pieces around the pilot fuel nozzle 22. Numeral 26 designates a main fuel supply port, which supplies the main fuel nozzles 21 with fuel 141. Numeral 27 designates a pilot fuel supply port, which supplies the pilot fuel nozzle 22 with pilot fuel 140.
Numeral 125 designates a cooling steam supply pipe for supplying therethrough steam 133 for cooling. Numeral 126 designates a cooling steam recovery pipe for recovering therethrough recovery steam 134 after being used for cooling of the tail tube 24 of the combustor. Numeral 127 designates a cooling steam supply pipe, which supplies therethrough cooling steam 132 from a tail tube outlet portion for cooling of the tail tube 24, as described later.
In FIG. 27(b), showing a portion of a wall 20a of the tail tube 24, there are provided a multiplicity of steam passages 150 in the wall 20a. Steam passing therethrough cools the wall 20a. In FIG. 27(c), a steam supply hole 150a and a steam recovery hole 150b are provided to communicate with the steam passages 150 so that steam supplied through the steam supply hole 150a flows through the steam passages 150 for cooling of the wall 20a and is then recovered through the steam recovery hole 150b.
In the combustor so constructed, the main fuel 141 is supplied into the eight pieces of the main fuel nozzles 21 from the main fuel supply port 26. On the other hand, the pilot fuel 140 is supplied into the pilot fuel nozzle 22 from the pilot fuel supply port 27 to be burned for ignition of the main fuel injected from the surrounding main fuel nozzles 21. Combustion gas of high temperature thus flows through the combustion tube and the tail tube 24 to be supplied into a combustion gas path of a gas turbine (not shown), and while flowing between stationary blades and moving blades, works to rotate a rotor. The combustor so constructed is arranged in various plural pieces according to the model or type, for example 16 pieces, around the rotor. The high temperature gas of about 1500.degree. C. flows in the outlet of the tail tube 24 of each of the combustors. Thus, the combustor 20 needs to be cooled by air or steam.
In the combustor of FIG. 27, a steam cooling system is employed. The cooling steam 132, 133, extracted from a steam source (not shown), is supplied through the cooling steam supply pipes 127, 125, respectively, to flow through the multiplicity of steam passages 150 provided in the wall 20a of the tail tube 24 for cooling of the wall 20a. The cooling steam then joins together in the cooling steam recovery pipe 126 to be recovered as the recovery steam 134 to be returned to the steam source for effective use thereof.
FIG. 28 is a view seen from plane K--K of FIG. 27(a) to show an outlet portion of the tail tube 24. Numeral 160 designates a combustion gas path, through which the high temperature combustion gas of about 1500.degree. C. is discharged. A flange 71 for connection to the gas turbine combustion gas path is provided at an end periphery of the outlet portion of the tail tube 24. FIG. 29 is a cross sectional view taken on line L--L of FIG. 28 to show a steam cooled structure of the tail tube outlet portion in the prior art. In FIG. 29, the multiplicity of steam passages 150 are provided in the wall 20a, as mentioned above, in parallel with each other. A cavity 75 is formed over the entire inner circumferential peripheral portion of the flange 71 of the tail tube 24 outlet portion and the multiplicity of steam passages 150 communicate with the cavity 75.
A manifold 73 is formed, being covered circumferentially by a covering member 72, between an outer surface portion of the wall 20a of the tail tube 24 and the flange 71. The respective steam passages 150 communicate with the manifold 73 via respective steam supply holes 74.
In the mentioned steam cooled structure, a high temperature combustion gas 161 of about 1500.degree. C., on the one hand, flows in the combustion gas path 160, and on the other hand, the temperature of air flowing outside of the manifold 73 within the turbine cylinder is about 400 to 500.degree. C. An inner peripheral surface portion of the wall 20a and that of the tail tube 24 outlet portion, which are exposed to the high temperature combustion gas 161, are sufficiently cooled by the cooling steam 132 flowing into the steam passages 150 from the manifold 73 via the steam supply holes 74. The steam in the cavity 75 cools also a portion 20b which is not exposed to the high temperature combustion gas 161 and the cooling steam 132 in the manifold 73 also cools a portion 20c. Hence, as compared with the inner wall 20a, the portions 20b and 20c are excessively cooled, causing a differential thermal stress between the wall 20a and the portions 20b and 20c, thereby causing unreasonable forces therearound, which results in the possibility of cracks occurring, etc.
The gas turbine combustor in the prior art as described above is what is called a two stage combustion type gas turbine combustor, effecting a pilot combustion and a main combustion at the same time. The pilot combustion is done such that fuel is supplied along the central axis of the combustor, and combustion air for burning this fuel is supplied therearound to form a diffusion flame (hereinafter referred to as a pilot flame) in the central portion of the combustor. Main combustion is done such that a main fuel premixture having a very high excess air ratio is supplied around the pilot flame so as to make contact with a high temperature gas of the pilot flame to thereby form a premixture flame (hereinafter referred to as a main flame). FIG. 30 is a conceptual view of such a two stage combustion type gas turbine combustor in the prior art.
With reference to FIG. 30, within a liner 252 of the combustor 20, the pilot fuel nozzle 22 for injecting a pilot fuel is provided along a central axis O' and a pilot air supply passage 256 is provided around the pilot fuel nozzle 22. The pilot swirler 31 for flame holding is provided in the pilot air supply passage 256. Further, the main fuel nozzles 21, main air supply passages 258 and the main swirlers 32 for supplying main fuel are provided around the pilot air supply passage 256.
The pilot cone 33 is provided downstream of the pilot fuel nozzle 22 and the pilot air supply passage 256. The fuel supplied from the pilot fuel nozzle 22 and the air supplied from the pilot air supply passage 256 effect a combustion in a pilot combustion chamber 262 formed by the pilot cone 33 to form the pilot flame as shown by arrow 266. The fuel supplied from the main fuel nozzles 21 and the air supplied from the main air supply passages 258 are mixed together in a mixing chamber 264 downstream thereof to form the premixture as shown by arrow 268. This premixture 268 comes in contact with the pilot flame 262 to form the main flame 270.
In the prior art combustor 20, as the pilot flame 266 and the premixture 268 come in contact with each other in a comparatively short time, the premixture 268 is ignited easily, whereby the main flame 270 burns over a comparatively short length in the axial direction or the main flow direction, and is thus liable to form a short flame. If the combustion is over such a short length, or in other words, in a narrow space, a concentration of energy released by the combustion in the space or a cross sectional combustion load of the combustor becomes high to easily cause combustion vibration. Combustion vibration is a self-induced vibration caused by a portion of the thermal energy being converted to vibration energy, and as the cross sectional combustion load of the combustor becomes higher, the exciting force of the combustion vibration becomes larger and the combustion vibration becomes more liable to occur. As mentioned above, in the prior art combustor, the combustion load is comparatively high and there is a problem that the combustion becomes unstable due to the combustion vibration.